The present invention refers to an axial bearing for a roller of a roller drill bit.
In a conventional roller drill bit e.g. for blast hole rock drilling the radial forces on the rollers are generally supported by a radial roller bearing and a sliding bush. The axial force is supported by a thrust bearing with a thrust washer. It can be mentioned that the axial force at a 121/4" drill bit with a rotational speed of 100 rpm amounts to about 40000-70000 N. The thrust bearing is commonly completely unlubricated and it is cooled by means of pressurized air.
In order to improve the service life of the thrust bearing it is earlier known to coat the end surface of the shaft journal on which the roller is arranged with a hard surface material, e.g. Stellite, which can be welded to the shaft journal and thereupon be ground. This end surface cooperates with a bottom surface of an aperture in the roller, which bottom surface is provided e.g. with a washer of highspeed steel, if desired with an insert of silver as a lubricant. These measures, however, do not prevent the hitherto known thrust bearings from having a comparatively short service life. They are rapidly worn out due to the unfavorable service conditions. A common wear is about 2-3 mm in 70 hours. The dimension change will cause large loads on other elements of the bearing assembly for the roller, which elements will be damaged. A high bearing temperature is generated in spite of the cooling with pressurized air flowing through the bearing, as the friction is high. The friction losses can amount to about 1500 W.
The aim of the present application is to provide a thrust bearing of the above mentioned kind, which bearing during operation is exposed to insignificant wear only and where the friction losses are small.